In sub-quarter-micron ultra-large scale integrated circuits (ULSIs), a low-resistivity gate electrode is required to reduce limitations on the speed of MOS devices due to the gate RC delay time. Tungsten (W)-poly (Si) gate structures have lower sheet resistance than conventional poly or polycide gates. Because W reacts with Si at temperatures as low as 600.degree. C., it is critical to have a high quality diffusion barrier between W and Si. WN and TiN are candidates as the diffusion barriers between W and poly-Si to avoid silicidation of the W film. In conventional post-gate-etch, dry/wet oxidation is used to remove the etch damage and to improve the gate dielectric strength. The gate materials including the metal materials (W and the barrier) are subjected to the oxidation. However, under selective oxidation conditions, such as the so-called wet hydrogen oxidation (WHO) procedures (Seichi Iwata et al., IEEE Transactions on Electron Devices, Vol. ED-31, No. 9, pp. 1174-1179, Sept. 1984), W-based materials will not be oxidized while TiN barrier is still subject to oxidation. Oxidation of the barrier TiN layer can result in lift-off of the W layer. Therefore, from a low-resistivity and process integration point of view, the W-poly gate electrode without TiN is preferred.
In one prior art process (Yasushi Akasaka et al., IEEE Transactions on Electron Devices, Vol. 43, No. 11, pp. 1864-1869, Nov. 1996), a PVD process is used to fabricate a W/WN/poly-Si gate electrode. WNx films were deposited on poly-Si by reactive sputtering a W target in a gas mixture consisting of Ar:N.sub.2 =1:1. W films were continuously deposited by DC magnetron. However, the application of this process to MOS devices with a serious topography is limited. due to the poor step coverage of PVD films. In addition, excess nitrogen in the WNx layer needs to escape from the stack during thermal treatment. Because the thermal treatment occurs after the deposition of the W film, the escaping nitrogen from the WNx layer can cause delamination.
Accordingly, there is a need for a method of forming a barrier layer between the poly and metal of a gate electrode that overcomes the above problems.